The structural characteristics of cellular phospholipid acyl chains required for ABCA1-mediated HDL formation.

Abstract

ATP-binding cassette protein A1 (ABCA1) mediates high-density lipoprotein (HDL) formation by transporting cellular cholesterol and phospholipids to apolipoprotein A-I (apoA-I). Although phospholipids serve as transport substrates for ABCA1 and the membrane constituents surrounding ABCA1, their roles in HDL formation remain unclear. Here, we elucidated the effect of the acyl chain structure of cellular phospholipids on HDL formation, particularly focusing on monounsaturated fatty acid (MUFA)-containing phosphatidylcholine (PC), the predominant phospholipid in most animal cells. PC molecules effluxed to apoA-I had an acyl chain composition similar to cellular PC, both being enriched in MUFA-containing species. Furthermore, manipulating the acyl chain composition of cellular PC by stealoyl-CoA desaturase inhibition or fatty acid supplementation led to similar changes in effluxed PC molecule composition. Thus, ABCA1 can transport various cellular PC molecules, including MUFA-containing species, without apparent preference for their acyl chain structure. Conversely, an appropriate acyl chain composition of cellular phospholipids is required for ABCA1 functional expression. Reducing MUFA content in the cellular phospholipids suppressed ABCA1 expression through two independent mechanisms: first, by inducing an endoplasmic reticulum (ER) stress response that decreases ABCA1 protein production; and second, by causing a folding defect in the ABCA1 protein, leading to immature glycosylation and failure of plasma membrane localization. Excess MUFA supply decreased ABCA1 expression without causing ER stress or defects in glycosylation and localization of ABCA1. Collectively, we revealed the contribution of MUFA-containing PC to HDL formation and identified the structural characteristics of cellular phospholipids required for their transport to apoA-I and functional expression of ABCA1.